1. Field of the Invention
This invention relates to nonaqueous electrolyte secondary batteries and methods for manufacturing the same.
2. Description of Related Arts
Nonaqueous electrolyte secondary batteries are presently widely used as secondary batteries having high energy density.
Exemplary positive electrode materials conventionally used in nonaqueous electrolyte secondary batteries are lithium-transition metal composite oxides, such as LiCoO2. Exemplary negative electrode materials used are carbon materials capable of storing and releasing lithium. Exemplary nonaqueous electrolytic solutions used are organic solvents, such as ethylene carbonate or diethyl carbonate, in which a lithium salt, such as LiBF4 or LiPF6, is dissolved as an electrolyte salt.
In recent years, mobile devices using nonaqueous electrolyte secondary batteries have increased power consumption such as because of their increasing range of functions. Therefore, there is strong demand for nonaqueous electrolyte secondary batteries having even higher energy density.
In order to realize a nonaqueous electrolyte secondary battery having a higher energy density, its positive-electrode active material must be increased in capacity. For this purpose, various high-capacity positive-electrode active materials and methods for manufacturing them have been proposed, for example, in JP-A-2009-32681, J. Electrochem. Soc, 149(8) (2002)A1083, J. Electrochem. Soc, 147(7) (2000)2478, and Solid State Ionics 144 (2001)263.
The crystal structure of a lithium-containing layered compound LiCoO2 presently widely used as a positive-electrode active material is an O3 structure belonging to space group R-3m. In relation to this lithium-containing layered compound LiCoO2, if lithium in the crystal structure is extracted about 60% by application of a potential of 4.5 V (vs. Li/Li+) or more, the crystal structure tends to break down to make the reversibility of the electrode reaction poor. Therefore, with the use of a lithium-containing layered compound belonging to space group R-3m, such as LiCoO2, the maximum possible discharge capacity density is about 160 mAh/g.
In order to further increase the discharge capacity density, the positive-electrode active material must be able to hold a stable structure even when a larger amount of lithium is extracted. One proposed method for manufacturing a lithium-containing layered compound having the above structure is to manufacture a lithium-containing layered compound by ion-exchanging a sodium-containing layered compound.
Specifically, for example, JP-A-2009-32681 describes a process for producing a lithium-containing oxide containing a minute amount of sodium by ion-exchanging part of sodium in a sodium-containing oxide with lithium. In addition, JP-A-2009-32681 also describes, as a lithium-containing oxide produced by the above process, a lithium-containing oxide belonging to space group P63mc and/or Cmca and represented by the composition formula LiANaBMnxCoyO2±α (where 0.5≦A≦1.2, 0<B≦0.01, 0.40≦x≦0.55, 0.40≦y≦0.55, 0.80≦x+y≦1.10, and 0≦α≦0.3).
This lithium-containing oxide described in JPA-2009-32681 is less likely to cause the crystal structure to break down even if a large amount of lithium is extracted from it by charging to a high potential. This literature states that, therefore, a high charge/discharge capacity density can be obtained by using the above lithium-containing oxide as a positive-electrode active material.